1. Field of the Invention
The field of the invention is Light Emitting Diode (LED) driver circuits, and more particularly, LED driver circuits with a boosted voltage output for driving an LED from a voltage power supply having a voltage that is lower than the turn xe2x80x9conxe2x80x9d voltage of the LED.
2. Description of the Related Art
LEDs are commonly used as status indicator lights and in numerical and alpha-numerical displays. LEDs are also used in opto-couplers to transmit optical signals between circuits. Such opto-couplers are typically used to provide a communications link between circuits that are electrically isolated from each other.
LEDs are typically driven by an integrated circuit using an LED driver circuit. This is because the output of many integrated circuits can not supply the current needed to turn on an LED. FIG. 1 shows a conventional LED driver circuit for driving an LED 100. The LED driver circuit comprises a bipolar transistor 115, a collector resistor 111, and a base resistor 120. The collector resistor 110, which is used to set the current level supplied to the LED 100, is connected between a voltage power supply 105 and the anode of the LED 100. The collector terminal of the transistor 115 is connected to the cathode of the LED 100, and the emitter terminal of the transistor 115 is connected to ground. One end of the base resistor 120 is connected to the base terminal of the transistor 115. The other end of the base resistor 120 is used for the input 122 of the LED driver circuit, which may be driven by the output of a logic gate in an integrated circuit (not shown). The base resistor 120 determines the input 122 impedance of the LED driver circuit and, thus, the drive requirement at the input 122.
Historically, the voltage power supplies used to power integrated circuits have had voltages greater than the turn xe2x80x9conxe2x80x9d voltages of many LEDs, which typically range from about 1.6 V to 3.6 V. This has allowed the use of the same voltage power supplies to power both integrated circuits and LEDs using conventional LED driver circuits. However, as the dimensions of integrated circuits have continued to be scaled down, the voltages of many voltage power supplies used to power integrated circuits have been reduced to values approaching the turn xe2x80x9conxe2x80x9d voltage of many LEDs. Recently, the voltage used to power many integrated circuits have been migrating from 5.0 V to 3.3 V. Ultimately, the voltage used to power many integrated circuits may be reduced to a value below 1 V. As a result, the voltage power supplies used to power many integrated circuits may eventually be unable to power LEDs using conventional LED driver circuits, thereby creating the need for separate voltage power supplies to power the LEDs.
Even as the voltages used to power integrated circuits remain above the turn xe2x80x9conxe2x80x9d voltage of many LEDs, the trend towards reduced voltages to power integrated circuits may lead to a degradation of the current control of conventional LED driver circuits. This can be illustrated by way of an example in which a 3.3 V voltage power supply 105 is used to power the LED driver circuit of FIG. 1. In this example, the 3.3V voltage power supply 105 has a maximum and a minimum worst case voltage of 3.6V and 3.0 V, respectively, and the LED 100 has a turn xe2x80x9conxe2x80x9d voltage of 2.5 V. The resistance of the collector resistor 10 is chosen such that the current supply to the LED 100 is about 10 mA at a nominal voltage power supply 105 voltage of 3.3 V. Assuming that the transistor 115 has a collector-emitter voltage drop of approximately 0.2 V, the current supply level to the LED 100 varies between 15 mA and 5 mA for the maximum and minimum worst case voltages of the voltage power supply 105, respectively. As a result, the LED driver circuit of FIG. 1 may exhibit poor current control to an LED 100 when the voltage of the voltage power supply 105 approaches the turn xe2x80x9conxe2x80x9d voltage of the LED 100.
Therefore, there is a need for an LED driver circuit having a boosted voltage output capable of driving an LED from a voltage power supply having a voltage that is lower than the turn xe2x80x9conxe2x80x9d voltage of the LED. This would allow an integrated circuit and the LED to be powered by the same voltage power supply when the voltage power supply has a voltage that is lower than the turn xe2x80x9conxe2x80x9d voltage of the LED. In addition, the expense of having to provide separate voltage power supplies to power the integrated circuit and the LED may be avoided. There is also a need for an LED circuit driver that exhibits good current supply control to an LED when the voltage of the voltage power supply approaches the turn xe2x80x9conxe2x80x9d voltage of the LED.
Embodiments of the present invention provide LED driver circuits with a boosted voltage output for driving an LED from a voltage power supply having a voltage that is lower than the turn xe2x80x9conxe2x80x9d voltage of the LED.
An LED driver circuit, built in accordance with one embodiment of the present invention, comprises an input buffer and a charge pump, both of which are connect to a voltage power supply having a voltage of Vdd. The charge pump boosts the voltage of the voltage power supply at its output in order to drive an LED having a turn xe2x80x9conxe2x80x9d voltage that is comparable to or greater than the voltage of the voltage power supply. The input buffer receives a control signal at its input and either enables or disables the charge pump based upon the received control signal.
In another embodiment of a preferred embodiment, the charge pump of the LED driver circuit comprises an oscillator, an inverter, a capacitor, and a diode. The oscillator has an input connected to the output of the input buffer and an output. The inverter has an input connected to the output of the oscillator and an output. The capacitor has a low electrode connected to the output of the inverter and a high electrode connected to the anode of the LED being driven. The anode of the diode is connected to the voltage power supply and the cathode of the diode is connected to the high electrode of the capacitor.
When enabled by the input buffer, the oscillator outputs a pulsing signal that alternately switches the inverter between a high output state and a low output state. When switched to the low output state by the pulsing signal, the inverter pulls the low electrode of the capacitor down to ground. The diode is forward biased and current flows from the voltage power supply to the high electrode of the capacitor through the diode. This current charges up the capacitor, raising the voltage at the high electrode of the capacitor to approximately Vddxe2x88x92Vt, where Vt is the potential drop across the diode. When switched to the high output state by the pulsing signal, the inverter raises the voltage at the low electrode of the capacitor to Vdd. This voltage rise at the low electrode of the capacitor causes the voltage at the high electrode of the capacitor to rise above Vddxe2x88x92Vt due to capacitive coupling. The high electrode of the capacitor rises until it reaches the turn xe2x80x9conxe2x80x9d voltage of the LED, at which point, the capacitor discharges through the LED, causing the LED to turn xe2x80x9conxe2x80x9d.
In another embodiment of a preferred embodiment, the diode of the charge pump is implemented using a diode-connected PFET. In the forward direction, the source of the PFET is connected to the voltage power supply, the drain of the PFET is connected to the high electrode of the capacitor, and the body of the PFET is connected to the drain of the PFET via a large value resistor. This type of body connection weakly forward biases the body-source junction of the diode connected PFET in the forward direction, thereby lowering the magnitude of the threshold voltage Vt of the PFET. By lowering the magnitude of the threshold voltage Vt in the forward direction, the potential drop across the diode connected PFET is decreased, thereby increasing the voltage boosting capability of the LED driver circuit.
In another embodiment of a preferred embodiment, a second charge-pump stage is connected to the LED driver circuit in order to increase the voltage boosting capability of the LED driver circuit. The second charge pump stage comprises a second inverter, a second diode, and a second capacitor. The second inverter has an input connected to the output of the first inverter and an output. The second capacitor has a low electrode connected to the output of the second inverter and a high electrode connected to the anode of the LED being driven. The anode of the second diode is connected to the cathode of the first diode and the cathode of the second diode is connected to the high electrode of the second capacitor.
In another embodiment of a preferred embodiment, the LED driver circuit drives two LEDs. The LED driver circuit according to this embodiment comprises a ring counter and a first and second NFET switch. Each one of the NFET switches is connected to the ring counter and one of the two LEDs being driven. During operation, the ring counter alternately switches on the first and second NFET switch to alternately turn xe2x80x9conxe2x80x9d the two LEDs.
In another embodiment of a preferred embodiment, a switch/current regulator is connected between the cathode of the LED being driven and ground in order to regulate the current level flowing through the LED. In one embodiment, the regulated current level is set by a reference current.
In another embodiment of the present invention, the LED driver circuit comprises a voltage regulator to regulate the boosted voltage output. In one embodiment, the voltage regulator is connected in series between the voltage power supply and the charge pump. When the voltage, Vdd, of the voltage power supply exceeds a predetermined voltage, Vddxe2x80x2, the voltage regulator reduces the voltage Vdd to the voltage Vddxe2x80x2, which is outputted to the charge pump. Otherwise, the voltage regulator passes the voltage, Vdd, of the power voltage supply to the charge pump. In another embodiment, the LED driver circuit regulates the boosted voltage output by controlling a charge-up voltage of first capacitor based upon a comparison of the charge-up voltage of the first capacitor and a reference voltage. In another embodiment, the LED driver regulates the boosted voltage output by controlling whether or not the inverter boosts the voltage of the first capacitor based upon a comparison of the power supply voltage, Vdd, and a reference voltage.
In another embodiment of the invention, the LED driver comprises an NFET switch, a first PFET switch, a second PFET switch and a capacitor. The gate of the NFET is driven by a clock signal B, the drain of the NFET is connected to the gate of the first PFET, and the source of the NFET is connected to ground. The source of the first PFET is connected to the voltage power supply, and the body of the first PFET is connected to the drain of the first PFET through a resistor. The gate of the second PFET is driven by a clock signal C, the drain of the second PFET is connected to the drain of the NFET, and the body of the second PFET is connected to the source of the second PFET 1215. The capacitor has a low electrode driven by a clock signal A, and a high electrode connected to the drain of the first PFET and the source of the second PFET. In this embodiment, current flows from the voltage power supply to the capacitor through the first PFET switch to charge up the capacitor.
In another embodiment of the present invention, the LED driver provides a continuous current to the LED being driven for applications in which the LED can not be switched xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d, i.e., pulsed.
In another embodiment of the present invention, the LED driver circuit of any one of the various embodiments of the invention and an LED are packaged together in an LED lamp package. The LED lamp package according to this embodiment includes a ground lead, an input control lead, and a voltage supply lead, wherein the ground lead includes a reflector dish. The LED lamp package further includes an LED driver chip comprising the LED driver circuit and an LED chip comprising the LED. Both the LED driver chip and the LED chip are mounted onto the reflector dish of the ground lead. The power supply lead is connected to the LED driver chip by a first bond wire, the input lead is connected to the LED driver chip by a second bond wire, and the LED driver chip is connected to the LED chip by a third bond wire. The top portions of the three lead are enclosed by a light transparent encapsulate.
In another embodiment of the present invention the LED lamp package does not include the input control lead. In this embodiment, the LED driver circuit on the LED driver chip is configured to automatically turn on when a sufficiently high voltage power supply is applied to the LED driver circuit.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.